1. Field of the Invention
This invention relates generally to perforating well casing, and in particular to an improved apparatus for perforating casing and fracturing multiple formations of highly deviated oil and gas wells in a single trip. More particularly, this invention relates to an improved electronic downhole release tool and actuation circuit therefore that is compatible with multiple select-fire perforating guns.
2. Background Art
Referring to FIG. 1A, after a well is drilled, the open borehole is typically lined; casing (2) is run into the borehole and cemented in place. In order to allow fluid ingress into the lined well, a perforating gun (14) is then lowered into the cased hole for forming openings (3) through the casing at selected locations within the subterraneous formations. The perforating gun (14) usually is made up of shaped charges that are detonated by a blasting cap. The blasting cap is activated by electrical current.
Perforating guns (14) are often deployed using wireline techniques, in which a wire rope or an armored electrical cable (8) is used to suspend the tool in the well bore. Occasionally, as illustrated in FIG. 1B, perforating guns (14) become stuck in the well bore due to warpage or burrs that occur during the detonation process. Because of this possibility, wireline systems typically include a weak link that can be sheared under a predetermined wireline tension so that the wireline (8) can be pulled free, leaving the downhole tools in the bore to be fished out by conventional means.
An improvement over the mechanical weak link is an electrically actuated release tool (10). The release tool (10) is located above the perforating gun (14) (and typically below a casing collar locator (13), sinker bars (12) and the wireline cable head (11)). The release tool (10) has two halves and uses current-activated explosive charges to shear a tension stud (21) to separate its upper portion (10A) from its lower portion (10B). In this manner, the wireline (8) and upper tool string above release tool (10) is, by electrical actuation, severed from the lower portion of the tool string below the release tool (10). The release tool (10) is designed so that when it is parted, the exposed upper end (20) of the lower tool string has a profile that facilitates the fishing process. As electric wireline perforating guns (14) are typically activated by positive DC voltage, the release tool (10) uses a negative DC voltage triggering source. Release tools (10) as described above are available from High Pressure Integrity, Inc. of New Orleans, La., for example.
In many wells it is desirable to perforate casing over greater distances in the wellbore than can be accommodated by one perforating gun. To avoid running perforating guns and withdrawing the spent charges from the wellbore repeatedly, it is advantageous to place a number of perforating charges or groups of charges in the well simultaneously. Select charges are individually fired as the perforating guns are moved along the cased borehole. This technique is called “select-fire,” and it is known in the art.
Examples of apparatus for selectively firing perforating charges are disclosed in U.S. Pat. Nos. 5,531,164 issued to Mosley on Jul. 2, 1996; 5,700,969 issued to Mosley on Dec. 23, 1997; and 7,387,162 issued to Mooney, Jr., et al. on Jun. 17, 2008, which are incorporated herein by reference. The electrical circuits in the devices are designed such that charges are fired sequentially by alternately applying negative and positive electrical voltages to the device.
Although most other electric wireline tools are powered with positive voltage, select fire perforating guns typically use both positive and negative voltage to selectively control the firing process. Thus, select-fire perforating guns have heretofore not been used with negative-voltage actuated electric release tools because of the conflicting operating voltages of the devices. Although additional conductors can be provided within the wireline to remedy conflicting voltages, such a solution is often not cost-effective, particularly given the capital costs in replacing existing wireline cable. As a result, if a select fire perforating gun becomes stuck down hole, wireline retrieval is generally limited to reliance on a weak point built into the system to allow the wireline to be pulled free by breaking the weak point.
However, when down hole tools are deployed in a highly deviated wellbore or a horizontal wellbore it may be extremely difficult to pull directly on the rope socket with sufficient force to part the weak link, because the wireline friction due to contact with the deviated bore hole becomes great. Therefore, as select-fire perforation becomes more prolific, it is desirable to provide the combination of an electric release tool with multiple-charge, select fire, perforating guns to provide for efficient perforation operations, particularly in highly-deviated well bores.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a method and apparatus for performing electric wireline operations, especially select-fire perforating operations, with the ability to activate a release tool using either positive voltage or negative voltage.
Another object of the invention is to provide a method and apparatus for allowing a wireline operator to release a select-fire perforating gun from the wireline by electronic means rather than mechanical pulling on the rope socket.